1. Field of the Invention
This invention relates to automatic focusing apparatus, and more particularly to an automatic focusing apparatus of the type which is suitable for optical video disks.
2. Description of the Prior Art
A generally-known field which requires automatic focusing relates to optical video disk play-back apparatus. Hereunder, therefore, this invention will be described by taking the automatic focusing in the optical video disk play-back apparatus as an example.
In order to play back information recorded on an optical video disk, it is necessary to cause a laser beam for play-back to precisely track an information track on the optical video disk and to focus the laser beam with high accuracy.
To this end, optical video disk play-back apparatus has heretofore employed a method wherein a light beam to be exclusively used for the tracking is generated separately from the light beam for the play-back of the video signals and a photodetector for detecting the tracking light beam is used for performing tracking control, and wherein a capacitance type detector, or an exclusive light beam and a corresponding photodetector, is/are employed for detection of focusing error so as to control the focusing. This method has the disadvantage that the construction of the apparatus becomes complicated.
In case where the use of a single light source is intended, as the employment of a plurality of light sources is expensive, it is necessary to form three light beams for the video signals, for the tracking and for the focusing. This brings about the disadvantage that the power of the single light source must be made very high.
On the other hand, there has been a method wherein a video signal and a focusing error signal are taken out with a single light beam. Herein, an optical element having a unidirectional lens action (hereinbelow, simply termed "cylindrical lens") is arranged in an optical system for detecting the video signal. By exploiting the astigmatic action of the light beam, a focusing error is detected in the form of changes in the shape of a reflected light beam and in the intensity distribution. The focusing control is conducted so that the intensity distribution of the reflected light beam may become constant.
This method is advantageous in that the apparatus is simple and, since the video signal and the focusing error signal are obtained with the single beam, the light source may be of low power. However, when the method is practically applied to an optical video disk play-back apparatus, it has the following disadvantage. Under the joint use with the tracking control and jitter control of the light beam as are indispensable to this sort of apparatus, the reflected light beam moves, and the movement becomes a disturbance to the focusing error signal, so that an accurate focusing control cannot be accomplished. As a result, the signal-to-noise ratio of the video play-back signal is low, and the play-back of a picture of high quality is impossible.
Referring now to FIGS. 1, 2, 3, 4A and 4B, description will be made of a typical prior-art apparatus and the disturbance which results in a problem in the functioning of the apparatus. FIG. 1 is a diagram for explaining the tracking control and the focusing control in a prior-art optical video disk play-back apparatus. In the figure, parts which are not relevant to the controls of the apparatus are omitted from the illustration. A light beam 30 emergent from a laser source (such as He-Ne laser) 10 passes through a condensing lens 11 and a beam splitter 12. After being reflected by a tracking mirror 13, the beam is converged on a point 31 on a video disk 22 as a convergent spot by a focusing objective lens 14. The video disk 22 is placed on a turntable 21, which is rotated at a constant speed by a motor 20. On the video disk 22, information tracks 23 bearing information relating to pictures, voices etc. are recorded at high density, which information is to be read with the convergent light spot.
At the convergent point 31 of the light beam, the convergent spot receives the information of the information track 23 in the form of changes in the reflection factor of the light. The reflected light returns to the objective lens 14 and the mirror 13, and it is separated from the entrance beam by the beam splitter 12. Then, the reflected light is guided to a photodetector 16 through a cylindrical lens 15. The light beam having reached the photodetector 16 becomes a detected spot 34. The output 40 of the photodector 16 includes, besides the video information read from the video disk 22, a signal relating to the converged state of the spot 31 of the light beam and a signal relating to the information tracking state of the beam. Therefore, the output 40 is separated into a focusing error signal 41, a tracking error signal 42 and the video information signal 43 by a signal distributor 17. The focusing error signal 41 is amplified by a focusing control amplifier 24, and the amplified signal is applied to a focusing control motor 25 to drive the objective lens 14 upwards or downwards, so that the focusing control of the spot 31 is accomplished. The tracking error signal 42, which turns the tracking mirror 13, is applied through a tracking control amplifier 26 to a motor 27, so that the tracking control of the spot 31 is accomplished. The video information 43 is applied to an amplifier 28, and is used for video play-back.
FIG. 2 shows the construction of a prior-art example of the photodetector 16 which forms the heart of the controls, including the detected spot 34 thereon, and the signal distributor 17. Owing to the action of unidirectional convergence of the cylindrical lens 15 placed in the path of the reflected light, the light spot 34 on the image of the photodetector 16 changes according to the change in the focusing image of the spot 31 on the video disk 22. At a correct focusing, the detected light spot becomes circular as indicated at 34. Accordingly, this focusing position is made the reference. When the video disk 22 comes close to the objective lens 14, the light spot is deformed as indicated at 34-1. On the other hand, when the video disk 22 moves away from the objective lens 14, the light spot is deformed as indicated at 34-2.
The photodetector 16 is quartered into individual photodetection portions having regions D.sub.1, D.sub.2, D.sub.3 and D.sub.4 formed by boundary lines 16-1 and 16-2. Signals are derived by the signal distributor 17 according to the shape into which the light spot 34 changes. Two adders 17-1 and 17-2 detect y-directional and x-directional components of the light spot, respectively. The focusing error signal 41 is evaluated by a summing amplifier 17-3.
Letting e.sub.F denote the focusing error signal, and D.sub.1s, D.sub.2s, D.sub.3s and D.sub.4s denote output signals of the respective photodetection portions D.sub.1, D.sub.2, D.sub.3 and D.sub.4, the following equation holds: EQU e.sub.F =(D.sub.1s +D.sub.2s)-(D.sub.3s +D.sub.4s) (1)
The focusing errors are accordingly detected as signals: EQU e.sub.F &gt;0
when the video disk comes close, and EQU e.sub.F &lt;0
when the video disk moves away.
The signal distributor means 17 further includes circuits for deriving the tracking error signal 42 and the video information signal 43. An adder 17-4 evaluates the total signal of the light spot 34. The tracking error signal 42 is obtained by applying the total signal to a low-pass filter 17-5, while the video information 43 is obtained by applying it to a high-pass filter 17-6. These signals 42 and 43 are respectively used for the tracking control and the video play-back.
With the construction as stated above, the recorded information on the video disk 22 can be played back. Ordinary video disks, however, involve a tracking movement of 100 to 250 .mu.m due to eccentricity etc., and a vertical movement of 100 to 500 .mu.m because of the inclination of the turntable, the bowing and thickness variation of the video disk, etc. On the other hand, the video disk 22 has information recorded thereon along the information tracks 23 minutely with high density in the order of .mu.m. In order to retrieve video information of good quality from such a video disk, the focusing of the spot 31 of the light beam and the tracking control of the spot to the information track must be at an accuracy of 1 .mu.m or higher. Further, needless to say, the focusing control and the tracking control must precisely operate compatibly.
Nevertheless, the focusing control and the tracking control have hitherto been often devised individually and independently. In a practical case of jointly using both the controls, therefore, it has been feared that additional problems would arise. FIG. 3 is a schematic view illustrating problems in the case where the astigmatism type focusing control as illustrated in FIG. 1 is used jointly with the tracking control. A path 30-31-34 of a light beam as depicted by solid lines indicates the reference state under which the light beam passes through the centers of the lens and the mirror as in FIG. 1. Now, suppose the mirror 13 is moved to a position 13' by the tracking control and the light spot 31 on the video disk 22 is thereby moved to point 31'. At this time, the light beam proceeds along a path indicated by the dotted lines from the spot 31' via the lens 14, mirror 13' and beam splitter 12 to the cylindrical lens 15. The light beam which enters the cylindrical lens 15 has a shape such as indicated at 32' and is projected as a detected spot 34' on the photodetector 16, at a position deviating from the center of the photodetector 16. This is attributed to the fact that, by exploiting the astigmatism of the lens 15, the photodetector 16 is situated at a point which is not a focal position.
FIG. 4A is a diagram which shows the relationship between the photodetector 16 and the light spot in greater detail. It illustrates the case where the light spot is in focus on the disk 22. At this time, the detected spot 34 becomes a circle. As to the detected spot 34 in the case where the tracking control is in the reference state, the focusing error signal e.sub.F becomes: EQU e.sub.F =0
and a correct focusing error signal is obtained. However, as to the detected spot 34' in the case where the light spot has been moved by the tracking control, the focusing error signal e.sub.F has a finite value even when the light spot is in focus. Now, consider the areas of the light spot 34' in the photodetection portions D.sub.1 -D.sub.4.
First, a straight line l passing through both the centers of the spots 34 and 34' is drawn. Subsequently, a circle 36' passing through the center of the spot 34 is depicted with its center at the center of the spot 34'. Auxiliary lines 38 and 39 are drawn in parallel with the boundary lines 16-1 and 16-2, respectively, so that they may pass through the point of intersection C between the circle 36' and the straight line l. The detected spot 34' divided by the auxiliary lines 38 and 39 (dotted lines) is reproduced in detail in FIG. 4B. In this figure, the sum between the area of an oblique line part of the spot 34' on the photodetection portion D.sub.3 and the area of an oblique line part on the photodetection portion D.sub.4 becomes equal to the area of an oblique line part of the detected spot 34' on the photodetection portion D.sub.1. The area of that part of the spot 34' on the photodetection portion D.sub.3 which is fully painted in black is equal to the area of that part of the detected spot 34' on the photodetection portion D.sub.2 which is fully painted in black.
Accordingly, a signal corresponding to a rectangle 37 appears in the focusing error signal e.sub.F represented by Eq. (1), and the signal e.sub.F becomes: EQU e.sub.F &lt;0
so that the correct focusing error signal is not produced. This indicates that the result of the tracking control has become a disturbance to the focusing control, and can be generally deemed one of the interaction problems in the multi-variable control.
The motion of the light spot 31 (shown in FIG. 3) due to the tracking control is a motion following a motion in the radial direction based mainly on the eccentricity of the information track. The motion of the detected spot on the photodetector as based on such motion of the light spot is a reciprocating motion on the x-axis. On the other hand, although omitted from FIGS. 1 and 3, a jitter control for compensating for a peripheral speed change of the information track is employed in association with the tracking control in the actual video disk play-back apparatus. In the jitter control, the light spot is oscillated in the circumferential direction orthogonal to the direction of the tracking control by means of another galvano-mirror. The motion of the detected spot on the photodetector as based on the oscillation becomes a reciprocating motion on the y-axis. Accordingly, the motion of the detected spot as based on a jitter correction is the reciprocating motion which shifts 90.degree. in phase relative to the reciprocating motion based on the tracking control. Consequently, the combined motion of the detected spot as based on both the tracking and jitter controls is fundamentally a circular motion, and the center of the detected spot depicts a circle on the photodetector 16 as indicated at 36 in FIG. 4A.
The disturbance which develops due to the tracking control and the jitter control in spite of the fact that the light spot 31 or 31' on the video disk is in focus is designated h.sub.F . Let it now be supposed that the intensity distribution of the detected spot is flat. Letting s denote the eccentric distance of the detected spot and .theta. denote the argument of the center of the detected spot as taken counterclockwise from the boundary line 16-1, the disturbance h.sub.F which is given by the area of the region 37 in FIG. 4A is represented by: EQU h.sub.F =I.sub.o (2s sin .theta.)(2s cos .theta.)=2I.sub.o s.sup.2 sin 2.theta. (2)
where I.sub.o indicates the intensity of the detected spot. In practical use, the intensity distribution of the light spot is considered to be gaussian. In that case, I.sub.o becomes a function of s. However, in a range in which the eccentric distance s is small, I.sub.o is little dependent upon s and can be approximated by a constant. Letting .omega. denote the rotational frequency of the video disk and t denote the period of time, EQU .theta.=.omega.t
Accordingly, EQU h.sub.F =2I.sub.o s.sup.2 sin 2.omega.t (3)
Since the rotational frequency of an ordinary video disk is 30 Hz, the fundamental frequency of the disturbance h.sub.F is 60 Hz. Although the value of the eccentric distance s varies in dependence on the tracking error and the characteristics of the optical system, it becomes 1/4 to 1/3 of the diameter of the light spot for a tracking error of 250 .mu.m in an example, and hence, the disturbance h.sub.F becomes a considerably great signal. The prior-art focusing control illustrated in FIGS. 1 and 2 regards this disturbance as the focusing error and operates so as to make e.sub.F zero. As a result, therefore, defocusing is induced. In an example in which the prior-art method is applied, the defocusing ascribable to this disturbance becomes 2 to 5 .mu.m, and it exceeds 0.5 to 1 .mu.m which is the desired accuracy of the focusing control requested in the ordinary video disk play-back apparatus. Consequently, when this disturbance is allowed to stand, the video disk is played back by a light beam which becomes out of focus periodically, and only play-back pictures of low quality can be obtained. In order to acquire play-back pictures of high quality, it is necessary to compensate for the disturbance and to thus enhance the accuracy of the focusing control.